What are hidden variables in quantum mechanics? I am aware there are many types but what exactly do they mean or even "do" exactly?

Do they mean that the quantum indeterminacy becomes hidden but still exists in reality somewhere but is hidden? Or does it mean that it is a hidden mechanism which means the reality underneath really is single world determinism or there abouts? I have done some searching but it really is not clear to me.

2 Answers
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Hidden variables is an idea among some classical physicists that Quantum Mechanics isn't really as random and unmeasurable as it appears but could be entirely predictable if we knew the internal rules that were being applied. These are the hidden variables.

I would say that it's a pretty much discredited idea and most modern physicists believe that QM really is as weird as it seems

ok thankyou, that answers my question. I had a comment on here once which said the variables "hide" the randomness and dont "get rid of it" this implyed that the randomness and indetermincy still existed in the universe it just wasent visible anymore. I knew the comment was misleading. Ron Maimon has also stated "Holographic hidden variables could conceivably even be holographically local, meaning that they are local on a holographic screen" the way he words this is as if the HV are some identity which hides quantum randomness rather than them being unknown physical laws
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user21847Mar 12 '13 at 18:35

I think the term 'classical physicists' is unnecessary. Firstly because there really aren't any of those left and secondly because it slightly comes across as condescending and almost dismissive. I'd rather say hidden variables are a possibility that has been disproved locally. Global hidden variables cannot be ruled out, but non-local theories are widely unappreciated mostly because they are non-local and locality is considered a big thing in physics.
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WouterMar 12 '13 at 22:40

Hidden variables are variables unaccounted for in a deterministic model of the universe. We can predict the number shown on a dice being rolled with greater accuracy if we know the roughness of the ground it comes into contact with, with greater accuracy. But generally we don't, and therefore it's a "hidden" variable, one of many responsible for the random behaviour of a rolled dice.

So it's not a thing or entity which takes the randomness of quantum mechanics and hides the randomness(superposition terms, entanglement etc) somewhere else in the universe?
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user21847Mar 12 '13 at 19:21

...so that it all still exists somehow but somewhere else?
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user21847Mar 12 '13 at 19:22

@user21847 our ignorance and inability to measure these variables hides them from us.
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Physiks loverMar 12 '13 at 19:24

Yes but the superstition terms as in a real wavefunction are not hidden somewhere else in the universe? Hidden away by these variables
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user21847Mar 12 '13 at 19:30

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This is not the meaning of "hidden variable" in the context of Bell's Inequality which is (I think) the most common use of it that would be tagged [quantum-mechanics] (and especially [quantum-interpretations]). In that context it means non-observables that nonetheless have definite values that make seemingly random behavior deterministic.
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dmckee♦Mar 12 '13 at 19:54